# Metre

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## Measurement

The metre (spelled "meter" in American English) is one of the seven SI base units. It is defined as the length of path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.

### History

The metre was originally defined in 1791 by the [French Academy of Sciences]? as 1/10,000,000 of the distance along the Earth's surface from the North Pole to the Equator along the meridian? of Paris. Uncertainty in the measurement of that distance lead the [International Bureau of Weights and Measures]? in 1889 to redefine the metre as the distance between two lines on a standard bar of platinum-iridium kept at Sevres?.

In 1960, as lasers had become available, the 11th [General Conference on Weights and Measures]? changed the definition of metre to be the length of 1,650,763.73 wavelengths in vacuum of the orange-red emission line in the spectrum? of krypton-86. In 1983 the General Conference on Weights and Measures defined the metre as the distance traveled by light in a vacuum in 1/299,792,458 of a second (that is, the speed of light in a vacuum was defined to be 299,792,458 metres per second). Since the speed of light is believed to be constant everywhere, a definition based on light is easier to maintain and more consistent than a measurement based on the circumference of the Earth or the length of a specific metal bar. Thus, should the bar be destroyed or lost, the standard meter can still be easily recreated in any laboratory.

## Literature

Metre is a term used in the scansion of poetry, usually indicated by the kind of feet and the number of them: for instance, "iambic pentameter", "dactylic tetrameter", etc. See meter in poetry.

## Music

Metre refers to the measurement of a musical line into units such as beats and measures, indicated in Western notation by a symbol called a time signature. Properly, "metre" describes the whole concept of measuring rhythmic units, but it can also be used as a specific descriptor for a measurement of an individual piece as represented by the time signature -- e.g., "This piece is in 4/4 metre" is equivalent to "This piece is in 4/4 time" or "This piece has a 4/4 time signature."

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